1. Field of the Invention
The present invention relates to a lookup table generation method and related device for a predistorter, and more particularly, to a lookup table generation method and related device for generating a plurality of predistortion parameters through a plurality of single tone signals sent to a channel simulation device.
2. Description of the Prior Art
A power amplifier is the most important component in radio frequency circuitry of a wireless communications system. A power amplifier with high output power usually exhibits output power saturation and low output linearity, which results in decreased output signal quality. It is therefore an important issue to avoid output signal distortion and enhance output linearity of the power amplifier.
The nonlinearity of a power amplifier relates to an input power of the power amplifier and can be realized by AM/AM and AM/PM characteristics shown in FIG. 1 and FIG. 2. As shown in FIG. 1, the AM/AM characteristic shows the nonlinearity of output amplitude to input amplitude. When input power exceeds a certain level, output power enters a saturation state and does not increase corresponding to the input power. As shown in FIG. 2, the AM/PM characteristic shows the nonlinearity of output phase to input amplitude. When the input power exceeds a certain level, output phase shifts according to the input power.
A digital predistortion method which performs digital signal processing on baseband signals through a predistorter reduces nonlinearity and signal distortion. Please refer to FIG. 3, which is a schematic diagram of a communications transmitter 30 using a digital predistorter according to the prior art. The communications transmitter 30 comprises a predistorter 300, a D/A converter 302, an up converter 304, a power amplifier 306 and an antenna 308. The predistorter 300 comprises an amplitude estimation unit 310, a lookup table 312 for storing predistortion parameters, and a predistortion unit 314. When a baseband signal is transmitted to the predistorter 300, the amplitude estimation unit 310 estimates amplitude of the baseband signal and outputs a value of the amplitude to the lookup table 312. The lookup table 312 decides a proper predistortion parameter for the estimated amplitude, then the predistortion unit 314 performs a predistortion process on the baseband signal according to the predistortion parameter, for generating a compensated signal. In other words, the predistorter 300 is utilized for compensating the baseband signal according to the predistortion parameter. The D/A converter 302 converts the compensated signal generated by the predistorter 300 to an analog signal. The up converter 304 transfers a frequency of the analog signal to a frequency appropriate for the antenna 308 to transmit. Finally, the power amplifier 306 amplifies the analog signal and the antenna 308 transmits the analog signal to the air.
In the predistorter 300, the predistortion parameters are designed according to AM/AM and AM/PM characteristics, so the compensated signal is able to improve output linearity of the power amplifier in advance. However, it is not efficient enough, thus closed loop delay of the power amplifier should be estimated before the predistortion parameters are generated.